Phosphors widely known are those using a silicate, a phosphate, an aluminate or a sulfide as a matrix material and a transition metal or a rare earth metal as a luminescent center. On the other hand, with respect to a white LED, attention is drawing toward phosphors excited by an excitation source with high energy such as ultraviolet or blue light to emit visible light, and development thereof is under way. However, the above-mentioned conventional phosphors have a problem that the luminance thereof decreases as a result of exposure to the excitation source. Recently, nitride and oxynitride phosphors are drawing attention as phosphors with small decrease in luminance, because they are materials having a stable crystal structure and being capable of shifting the excitation light and luminescent light to the longer wavelength side.
Among the nitride and oxynitride phosphors, an α-sialon activated by a specific rare earth element is known as a material having a useful fluorescence property and its application to a white LED or the like is being studied (cf. Patent Documents 1 to 5 and Non-patent Document 1). In addition, it is found that Ca2(Si,Al)5N8, CaSiAlN3 and a β-sialon activated by a rare earth element also have the same fluorescence property (cf. Patent Document 6 and Non-patent Documents 2 and 3). Other phosphors proposed include phosphors of nitrides and oxynitrides, such as aluminum nitride, magnesium silicon nitride, calcium silicon nitride, barium silicon nitride, gallium nitride and zinc silicon nitride (which will also be referred to hereinafter as nitride phosphors and oxynitride phosphors in the order named).
The α-sialon is obtained by firing a powder mixture composed of silicon nitride, aluminum nitride, aluminum oxide, an oxide of an element penetrating into a crystal lattice to make a solid solution therewith, etc., at a high temperature in nitrogen. A variety of fluorescence properties can be obtained depending on a ratio of silicon nitride and the aluminum compounds, a kind of the element penetrating to make a solid solution, a ratio of the element becoming the luminescent center, and the like. The element penetrating to make a solid solution is Ca, Li, Mg, Y or a lanthanide metal (except for La and Ce) and the α-sialon takes such a structure in order to maintain electrical neutrality that Si—N bonds are partially substituted by Al—N bonds and Al—O bonds. When a part of the element penetrating to make a solid solution is the rare earth element as the luminescent center, the fluorescence property is exhibited.
Incidentally, it is still the case that the white LEDs obtained heretofore are inferior in luminous efficiency to fluorescent lamps, and there is a strong demand for a LED superior in luminous efficiency to the fluorescent lamps, particularly, a white LED from the viewpoint of energy conservation in industry.
A combination of plural colors is required to make white light, different from monochromatic light, and a general white LED is composed of a combination of an ultraviolet LED or a blue LED with a phosphor emitting visible light with the LED as an excitation source. Thus, in order to improve the efficiency of the white LED, it is necessary to improve not only the luminous efficiency of the LED itself, the ultraviolet LED or blue LED, but also the luminous efficiency of the phosphor used therein, and to improve an efficiency of extraction of emitted light to the outside. Improvements in all these efficiencies are necessary to broaden application of the white LED, including application for general illumination.
One of the known methods for synthesizing such phosphors, for example, in a case of α-sialon powder, is a reduction and nitridation method wherein a powder mixture of aluminum oxide (Al2O3), silicon oxide (SiO2), an oxide of a metal or an element capable of making a solid solution in the lattice, and so on is subjected to a heat treatment in a nitrogen atmosphere in the presence of carbon (cf. Non-patent Documents 4 to 6).
This method has merits that raw-material powders are inexpensive and the synthesis can be carried out at a relatively low temperature around 1500° C. However, this method goes through plural intermediates on the way of the synthesis and generates gas components such as SiO and CO, and it is thus difficult to obtain a phosphor in a single phase, to severely control the composition, and to control particle sizes.
The α- or β-sialon powder can be obtained by firing a mixture of silicon nitride, aluminum nitride, an oxide of a metal or an element capable of making a solid solution in the lattice thereof, and so on at a high temperature and pulverizing the obtained sintered body. However, there was a problem that the luminescence intensity of the phosphor was decreased by the pulverizing operation of the sintered body as described below.
Furthermore, the phosphor for the white LED is generally used as dispersed in the form of micron-size particles in a sealant such as epoxy. The dispersion state and compatibility with the sealant, however, significantly affect the efficiency of extraction of LED light. Furthermore, surface coating of the phosphor is also being studied for improvement in the luminous efficiency of the phosphor and prevention of degradation, but its effect is different depending on the phosphors; there is a case not showing any effect at all and the effect was thus uncertain (Patent Documents 7 to 9).
The nitrides and oxynitrides have larger refractive indices and smaller specific gravities than the oxide phosphors such as YAG:Ce conventionally used for the white LEDs, and they failed to provide a satisfactory luminescence property as an LED when used in the same manner as the oxide phosphors.    Patent Document 1: JP-A-2002-363554    Patent Document 2: JP-A-2003-336059    Patent Document 3: JP-A-2003-124527    Patent Document 4: JP-A-2003-206481    Patent Document 5: JP-A-2004-186278    Patent Document 6: JP-A-2004-244560    Patent Document 7: JP-A-2001-303037    Patent Document 8: JP-A-2000-204368    Patent Document 9: International Publication WO96/09353 Pamphlet    Non-Patent Document 1: J. W. H. van Krebel, “On new rare-earth doped M-Si—Al—O—N materials,” TU Eindhoven, The Netherlands, p. 145-161 (1998)    Non-Patent Document 2: Extended Abstracts (The 65th Autumn Meeting, September, 2004, Tohoku Gakuin University) No. 3, p. 1282-1284; The Japan Society of Applied Physics    Non-Patent Document 3: Extended Abstracts (The 52nd Spring meeting, March, 2005, Saitama University) No. 3, p. 1615; The Japan Society of Applied Physics and Related Societies    Non-Patent Document 4: M. Mitomo et al., “Preparation of α-SiAlON Powders by Carbothermal Reduction and Nitridation,” Ceram. Int., 14, 43-48 (1988)    Non-Patent Document 5: J. W. T. van Rutten et al., “Carbothermal Preparation and Characterization of Ca-α-SiAlON,” J. Eur. Ceram. Soc., 15, 599-604 (1995)    Non-Patent Document 6: K. Komeya et al., “Hollow Beads Composed of Nanosize Ca α-SiAlON Grains,” J. Am. Ceram, Soc., 83, 995-997 (2000)